Solidarity means that we are interested in the good of all, even of people nobody thinks about because they have no voice and no power. The goods of the earth are there for everyone. The common good consists not only of the material or external good of all human beings; it also includes the comprehensive good of the human being, including even the spiritual good.
The common good of society is not an end in itself. It is only part of a bigger picture, the ultimate end of which is God.